Recently in the field of the image forming technology based on electrophotography such as copiers and printers, required has been a technology of a level which enables an accurate reproduction of a microdot image of 1200 dpi (in the present invention, “dpi” indicates the number of dots per 2.54 cm) in accordance with the development of the digital technology.
In order to faithfully reproduce the microdot image, miniaturization of the diameter of a toner particle has been studied, and attracted attention has been a chemical toner represented by a polymerization toner to which various types of controls can be applied in the manufacturing process, and, as a result, toner with a small diameter for faithfully reproducing the microdot image has been obtained (for example, see Patent Document 1).
On the other hand, as a method for fixing a toner image transferred on an image forming substance (also referred to as a transfer material) such as paper, a fixing method based on contact fixing, for example, a thermal roll fixing method and a thermal belt fixing method, is widely used. However, the contact fixing method tends to cause an offset problem in which melted toner adheres onto a heat member and the adhered toner is transferred to another image forming substance.
As the method to prevent the offset problem, for example, there has been a method to add a releasing property to the heat member by coating silicon oil on a surface of the heat member of a fixing device. However, the transfer material (image forming substance) coated with silicon oil becomes unrewritable with a writing tool such as a ball-point pen, and the transfer material may be contaminated by a volatile component contained in the silicon oil, both of which are not favorable for a business tool.
To cope with the above problem, development of an oil-less toner in which a releasing property is added to the toner particle itself has been carried out, and appeared has been a technology to add a fixing improver in the toner particle to provide the toner with a releasing property. To provide a releasing property, for example, an oil-less chemical toner containing a compound such as an ester compound of higher fatty acid having a long chain hydrocarbon group has been developed (for example, see Patent Documents 2, 3). The appearance of the toner containing the ester compound as the release agent has greatly contributed to the development of the oil-less image formation technology.
Recently, for the image formation technology based on the electrophotographic method, there has been a demand to reduce the electric consumption of printers or copiers in order to reduce the influence to the environment in association with the image formation and the business costs in the office. As one of the measures to achieve the above demand, a technology to lower the fixing temperature of such electrophotographic image compared to the current fixing temperature has attracted attention. Further, because of the characteristic that the required number of sheets can be printed at required time, an image forming apparatus based on electrophotography has been extensively used in the field of short run printing.
However, when fixing was carried out at a lower temperature using the toner-containing the above ester compound, the formed toner image was found to have a tendency to easily peel from the transfer material. Further, when a large amount of printing was continuously carried out, for example, a several hundred-thousand-sheets of printing was repeated, the charge rising capability of the toner showed a tendency to degrade, accordingly, durability of the toner has been desired, by which the charge rising capability did not vary even when a large amount of image formation was repeated. As described above, there has been a demand to provide a property, which meets the new movements or the needs of the market, to an oil-less toner.
Patent Document 1: Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 2000-214629 (see Paragraph 0061 and other paragraphs)
Patent Document 2: JP-A No. 2002-287405 (see Paragraph 0051 and other paragraphs)
Patent Document 3: JP-A No. 2003-91101 (see Paragraph 0208 and other paragraphs)